In an image forming apparatus such as a copying machine or a printer, there is a fixing device in which, for the purpose of improving an environmental property, a belt member is used to widen a nip width between a heating member and a pressure member in order to start the apparatus at a high speed and to perform fixing at a low temperature. In the fixing device in which the belt member is used to widen the nip width, there is a fear that toner is shifted on a sheet during fixing by the deflection, floating or the like of the belt member, and an image is blurred.